In order to determine the distance between the slits in the Fresnel biprism experiment, on...

In order to determine the distance between the slits in the Fresnel biprism experiment, one puts a convex lens in between the biprism and the eye piece. Show that if D > 4 f one will obtain two positions of the lens where the image of the slits will be formed at the eye piece; here f is the focal length of the convex lens and D is the distance between the slit and the eye piece. If d1 and d2 are the distances between the images (of the slits) as measured by the eye piece, then show that . What would happen if D < 4 f ?